1. Field of Invention
The present invention relates to an optical recording medium with a multitude of information recording layers and particularly relates to an interlayer spacing structure of a multilayer optical disc provided with five or more information recording layers or a cover layer and four or more information recording layers.
2. Description of the Related Art
In recent years, the use of optical discs in audio field, video field, computer field and many other fields has been promoted since a large quantity of information signals can be recorded at a high density. Recording capacity required for such optical discs increases year after year and various methods have been and are being proposed to accomplish this. A method for forming an information recording layer (hereinafter, “recording layer”) of an optical disc into a double layer structure has been proposed as one of them and already put to practical use.
Such optical discs are structured to laminate recording layers via a transparent intermediate layer, and optical discs with three or more recording layers have also been proposed. However, multilayer optical discs have a problem that reflected light from a desired recording layer (hereinafter, “readout layer”) having information reproduced therefrom is mixed with reflected lights from recording layers (hereinafter, “other layers”) other than the readout layer, and some of the reflected lights from the other layers have large influence due to the spacings between the respective recording layers.
FIGS. 24 and 25 are diagrams showing this problem, wherein FIG. 24 is a perspective view partly cut away showing the external appearance of an optical disc and FIG. 25 is an enlarged partial section showing a part PA shown in FIG. 24.
In FIGS. 24 and 25, the optical disc having a plurality of recording layers is identified as reference element 100, recording layers are identified as reference elements 101-103, a supporting substrate is identified as reference element 104, intermediate layers having an equal thickness are identified as reference elements 105 and 106, a cover layer is identified as reference element 107, a light incident surface is identified as reference element 107a, and light irradiated from an optical head is identified as reference element 108.
A case of reproducing a signal on the recording layer 101 from the optical disc 100 structured as above is thought. At this time, the light 108 irradiated from the optical head is focused on the recording layer 101, but a part of the light reflected by the recording layer 102 is also focused on the recording layer 103 if the thickness of the intermediate layer 105 and that of the intermediate layer 106 are equal. A part of the light focused on this recording layer 103 is reflected by the recording layer 103 and reflected again by the recording layer 102 and returns to the optical head substantially along the same optical path as the light reflected by the recording layer 101. Thus, a problem of mixing a signal of the recording layer 103 with a reproduction signal occurs. Hereinafter, this is called a “back focus problem” in this specification.
Spacings between recording layers of an optical disc in consideration of such a back focus problem are proposed in patent Literature 1. In this literature, an occurrence of the above problem is prevented by constructing the optical disc such that spacings from an arbitrary recording layer to all the recording layers at a side toward a supporting substrate and spacings from this recording layer to all the recording layers at a side toward a cover layer do not equal each other. As an embodiment of this, a structure for making the thicknesses of the intermediate layers successively larger or successively smaller from the supporting substrate side toward the cover layer side has been proposed.
However, what is considered in the disc structure disclosed in patent literature 1 is only that light irradiated from the optical head is focused on the other layer during the reproduction and this focused light is mixed with the reproduction light to increase interlayer crosstalk, but the interference of the reflected light from the readout layer and reflected lights from the other layers is not considered.
For example, in the disc structure as shown in FIG. 25, a part of the reflected light from the other layer returns to the optical head substantially with the same wave front along the same optical path as the reflected light from the readout layer. Such light has high coherency and forms a light-dark distribution on a light receiving element due to interference. Further, this light-dark distribution varies according to a phase difference change from the reflected light from the other layer caused by the thickness variation of the intermediate layers in the optical disc. This becomes a serious problem of drastically reducing the qualities of a servo signal and a reproduction signal.
Patent Literature 1:
Japanese Unexamined Patent Publication No. 2001-155380